Understanding limitations to invasions by non-native plants at high latitudes

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Invasions by non-native plants are a serious problem throughout temperate North America. In contrast, northern boreal forests are relatively uninvaded, and the tundra remains almost free of invasive plants. Few studies have attempted to identify the factors responsible for the exclusion of non-natives from northern ecosystems, or to understand whether future environmental changes may increase their vulnerability to invasion. Poleward range limits of non-native plants may be set by barriers such as harsh environmental conditions, by reduced ability to tolerate competition or herbivory, or simply by limited dispersal. Townsites potentially can alleviate many of these factors. Frequent introduction and local movement of propagules may help to maintain populations of non-natives. Soils may be improved by nutrient enrichment and changes in soil biota. Human activities and structures may provide shelter and create warmer microclimates. Repeated disturbance may create sites for establishment and reduce competition by native plants. Finally, towns may provide refuges from native herbivores. Churchill, Manitoba (58.5º N), at the boreal/tundra boundary, provides a unique opportunity to study northern invasions. Over a hundred non-native plants have been recorded in town and associated sites, reflecting the area's long history of European occupancy and its role as a transportation hub, particularly for wheat and other seed crops. Many species have been able to persist for decades, but crucially almost none have invaded surrounding natural areas. I propose to investigate factors restricting locally-occurring non-natives to settled areas, and excluding them from nearby tundra and treeline habitats. To do this, I will combine a series of field observations and experiments in invaded areas in and near the Churchill townsite and in nearby uninvaded boreal and tundra environments. My lab will use transplants, common gardens, and experimental manipulations to test whether invasion is limited by failure of locally established invaders to successfully disperse to new sites, whether environmental factors including temperature, and soil nutrients may prevent successful establishment following dispersal, and whether biotic interactions including herbivory, competition, and soil feedback may prevent establishment even if physical conditions are favourable. We also will investigate whether conditions in town and other inhabited areas can remove these barriers. Results will help to determine whether northern invasions are an emerging problem, and to predict whether climate change will remove barriers to further spread of invaders.